docs: def add_nodes(self, nodes, parent=None, mode='horizontal'):
          ''' Adds one or more nodes to the current graph.
  
          Args:
              nodes (list): A list of nodes to add. Each element must be one of
                  the following:
  
                  * A dict containing keyword args to pass onto to the Node init.
                  * An iterable containing 1 - 3 elements. The first element is
                    mandatory, and specifies the Transformer at that node. The
                    second element (optional) is an iterable of child nodes
                    (specified in the same format). The third element
                    (optional) is a string giving the (unique) name of the
                    node.
                  * A Node instance.
                  * A Transformer instance.
  
              parent (Node): Optional parent node (i.e., the node containing the
                  pliers Transformer from which the to-be-created nodes receive
                  their inputs).
              mode (str): Indicates the direction with which to add the new nodes
                  * horizontal: the nodes should each be added as a child of the
                    'parent' argument (or a Graph root by default).
                  * vertical: the nodes should each be added in sequence with
                    the first node being the child of the 'parnet' argument
                    (a Graph root by default) and each subsequent node being
                    the child of the previous node in the list.
          '''
          for n in nodes:
              node_args = self._parse_node_args(n)
              if mode == 'horizontal':
                  self.add_node(parent=parent, **node_args)
              elif mode == 'vertical':
                  parent = self.add_node(parent=parent, return_node=True,
                                         **node_args)
              else:
                  raise ValueError("Invalid mode for adding nodes to a graph:"
                                   "%s" % mode)